Secondary batteries containing a non-aqueous electrolyte (non-aqueous electrolyte secondary batteries), particularly lithium ion secondary batteries having a high voltage and a high energy density are vigorously studied in recent years. Most of the currently available lithium secondary batteries contain, as a positive electrode active material, LiCoO2 that exhibits a high charge/discharge voltage. As the non-aqueous electrolyte, a non-aqueous solvent dissolving a solute therein is typically used. Usually, the non-aqueous solvent includes a cyclic carbonic acid ester, non-cyclic (linear) carbonic acid ester, cyclic carboxylic acid ester or the like, and the solute is lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) or the like.
Demand for higher capacity non-aqueous electrolyte secondary batteries is strong, and significant research and development effort has been placed on higher capacity positive electrode active materials that can replace LiCoO2. Particularly, nickel-containing lithium composite oxides composed mainly of nickel (e.g., LiNiO2) are studied intensively, and some have already gone into actual use.
High reliability and longer life of non-aqueous electrolyte secondary batteries are also in strong demand. Batteries containing LiNiO2, however, usually have lower cycle characteristics and lower thermal safety than those containing LiCoO2. As such, batteries containing LiNiO2 are not dominant in the market.
Under the circumstances, improvement of nickel-containing lithium composite oxides is being made actively. Japanese Laid-Open Patent Publication No. Hei 5-242891, for example, proposes LiaMbNicOdOe, where element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, V, Cu, Mg, Ti, Zn and Mo, 0<a<1.3, 0.02≦b≦0.5, 0.02≦d/c+d≦0.9, 1.8<e<2.2, and b+c+d=1. It is disclosed that the crystal structure of this active material is unlikely to change so that a high capacity can be achieved and the thermal stability can be improved.
Attempts are also made to add various additives to positive electrode active materials, negative electrode active materials and non-aqueous electrolytes. Japanese Laid-Open Patent Publications Nos. 2003-132950 and 2004-139963, for example, propose to add a fluorine atom-containing aromatic compound to a non-aqueous electrolyte. The object of Japanese Laid-Open Patent Publication No. 2003-132950 is to improve charge/discharge cycle characteristics. It is disclosed that a fluorine atom-containing aromatic compound is adsorbed on or reacted with the surface of a negative electrode so as to form a film, whereby the side reaction between the non-aqueous electrolyte and the negative electrode active material is inhibited. Japanese Laid-Open Patent Publication No. 2004-139963 discloses to use a fluorine atom-containing aromatic compound so as to reduce the amount of gas generated during continuous charging.
As described above, although improvement of nickel-containing lithium composite oxides is being made, those that can exhibit satisfactory cycle characteristics have not been achieved yet. The mere addition of a fluorine atom-containing aromatic compound to a non-aqueous electrolyte does not sufficiently improve cycle characteristics, either. It has been acknowledged that the side reaction between a non-aqueous electrolyte and a positive electrode active material becomes intense particularly in a high temperature environment, resulting in highly degraded cycle characteristics.
Japanese Laid-Open Patent Publication No. 2003-132950 is intended to inhibit the side reaction between a non-aqueous electrolyte and a negative electrode active material, and therefore it does not provide any effective proposal to inhibit the side reaction between a non-aqueous electrolyte and a positive electrode active material during the repetition of charge and discharge at high temperatures.
Likewise, Japanese Laid-Open Patent Publication No. 2004-139963 is intended to reduce the amount of gas generated during continuous charging (static trickle charge) for batteries containing a typical positive electrode active material, namely, LiCoO2. Accordingly, it does not provide any effective proposal to inhibit the side reaction between a non-aqueous electrolyte and a positive electrode active material during the repetition of dynamic charge and discharge at high temperatures.